This invention relates to the continuous manufacture of sheets of material, particularly wood chip board in general and more particularly to an improved manner of conveying the material used in the manufacture to a press.
A method for the continuous manufacture of sheets of material, especially of wood chip board in which a bed of self-bonding particles provided with a binder is applied to a first endless, revolving, substantially horizontal belt and is transferred from this belt to a second endless, revolving, substantially horizontal belt following the first belt, the second belt together with an endless belt disposed parallel and on top thereof and revolving in the same direction forming a press section in which the bed of particles is compressed between the belts and solidified under the action of pressure, and optionally heat to form a sheet of material, is described in U.S. patent application Ser. No. 55,510.
Another machine for the manufacture of chip board, fiber board and the like, in which the bed of chips is transferred from one conveyor belt to an endless revolving belt following the former, is known from DE-AS No. 22 31 802. This machine, however, does not comprise a continuously operating pressing section in which the material is compressed between endless, revolving belts, but a platen press which operates intermittently. The cake of fiber material provided with glue, from which the chip board is made, is fed to the platen press by means of a belt arrangement. For transferring the material from the belt arrangement into the platen press, an auxiliary device is provided which consists of a knife-like deflection strip, around which the conveyor belt is deflected with a very small radius, and in which the transfer point can be brought very close to the following belt or the belt trays of the platen press. Such small radii are possible, of course, only with correspondingly flexible belts, for instance, such of textile materials, but not with sheet metal belts.
However, in a chip board press of the type disclosed in U.S. Pat. No. 3,851,685 which utilizes sheet metal belts and which is the type of press with which the present invention is concerned, such small radii cannot be used.
In this press, the web is conducted between endless belts which are arranged on top of each other, co-rotate in the direction corresponding to the travel direction of the web and extend over the width of the web. Roller chains are provided which roll between the belts and a smooth support structures provided above and below the web. These roller chains revolve in endless succession in the longitudinal plane perpendicular to the web, are narrow relative to the width of the web, have constant width in the travel direction, form many individual legs closely adjacent transversely to the web and transmit the working pressure and, possibly, the heat from the support structure to the belts while rolling. The press section proper corresponds to the length of the support structure. However, the lower belt is longer and extends out of the press section up to the strewing device, in which the glue-covered chips are strewn onto the top side of the lower forming belt in a bed which results in the chip board. The lower belt thus takes along the bed of which the board is made and introduces it into the press section proper. In the region ahead of the press section, the lower belt therefore has the function of a conveyer belt, while the formation of the web proper takes place only in the press section.
The belts are steel belts about 1.2 mm thick. In the region, in which the chip material is transported on the lower belt before it enters the press section, the temperature of the lower forming belt must be low and in any case remain below 50.degree. C. so as to prevent premature condensation of the resin on the chips, which would result in reduced strength of the chip board. The web thus enters the press section in relatively cold condition and is heated up only in the press; the heat is transferred into the web through the roller chains bracing the belt against the support structure, and through the belt itself.
Because the belt is heated up when passing through the press section, the belt expands in the lengthwise direction in accordance with its temperature rise. The rollers of the roller chains rolling on the belt are taken along in the process. However, they are held on the other hand by the contact with the support structure on the side opposite the belt and further by the pins provided at the straps of the roller chains, so that the rollers of the roller chains cannot follow the expansion of the belts when passing through the press section.
This results in a longitudinal stress in the strap chains of the roller chains and in a tendency of the rollers of the roller chains to slip through in the lengthwise direction relative to the belt and/or the support structure.
Accordingly, one strives to avoid a temperature change of the belt arrangement when it passes through the press section, to the extent possible. Since it is impossible to make the temperature uniform over the entire length of the belt, because it must not exceed a given value in the forward section (which, in some cases, may even require cooling the belt after it leaves the press section), the approach remains to separate the belt arrangement and to use a forward traveling belt which has only a transport function and extends up to the entrance of the press section, and to leave the belts in the press section entirely in the press so that they have a uniform, high temperature, without thereby influencing the temperature of the conveyor belt in the forward-travel section.
The real problem, however, is now the transition from the first belt to the belt in the press section. The bed is carefully built up from different layers of chips with different fineness and different glue application, where the finest, i.e. practically meal-fine chips are found in the outer layers and drop through any gap in the belt arrangement. If the transition is not perfect, the composition of the bed also gets mixed up.
The design according to DE-AS No. 22 31 802, where the belts are conducted around a sharp corner, is out of the question in a press according to U.S. Pat. No. 3,851,685 since the sheet metal belts used require a certain minimum radius for the deflection.
Thus, it is the object of the present invention, starting with a press of the type disclosed in U.S. Pat. No. 3,851,685, to provide a method which permits a transition from a first belt to the belt in the press section in such a manner that there is as little disturbance as possible of the material transported on the belts.